In a telecommunication connection between a transmitter and a receiver, a baseband signal modulates a carrier signal to result in a modulated carrier signal, the transmitter sends the modulated carrier signal along a transmission medium to a receiver, then the receiver demodulates the modulated carrier signal to retrieve the baseband signal. A multiplex of several baseband signals may modulate the carrier signal.
Due to modulation, the frequency spectrum of the carrier signal spreads over a frequency band, the width of which has traditionally been called “bandwidth”. The bandwidth is measured in cycles per second (or Hertz). However, when the baseband signals modulating the carrier represent digital data, the total bit rate that can be accommodated within the bandwidth is also, colloquially, referenced as a bandwidth and measured in bits per second.
A transmission medium may accommodate several modulated carrier signals, each carrying a multiplex of baseband signals, and the portion of the frequency spectrum accommodated by the transmission medium allocated for each modulated carrier is called a channel. In other words, the transmission medium may carry several channels, each channel occupying a portion of the total frequency spectrum of the transmission medium. Naturally, the channels occupy non-overlapping portions of the spectrum. In a fiber-optic link, the frequency band allocated to each channel is often associated with the wavelength of the carrier signal and called a “wavelength channel” or, simply, a wavelength.
If the transmission medium traverses a switching node, where the baseband signals within the bandwidth of a modulated carrier signal are to be directed to different destinations, the switching node must first demodulate the modulated carrier signal, retrieve the individual baseband signals, and direct each of the retrieved baseband signals to an output port of the switching node that is associated with the destination of the particular baseband signal. Then, a multiplex of baseband signals received at each output port modulates a carrier signal leading to a subsequent switch. The process where the baseband signals within a single carrier are detected and routed individually, is essentially a baseband switching process performed by a baseband switch. A baseband switch offers fine-granularity and, with the current state of the art, is based on an electronic switching fabric with electronic data buffers provided at the input ports and output ports.
If the carrier signal shares the transmission medium with several other carrier signals, forming a multiplex of carrier signals, each carrier signal carrying a multiplex of baseband signals, a “coarse” switch may separate the individual modulated carrier signals and direct each modulated carrier signal to a designated transmission medium. A coarse switch switching entire modulated carrier signals is hereinafter called a carrier switch.
The task of retrieving baseband signals is not required in a carrier switch and, therefore, a carrier switch is less complex than a baseband switch for the same capacity. However, a carrier switch, being a very coarse switch, may force a given modulated carrier signal to traverse multiple carrier switches (hops). In contrast, baseband signals from several baseband switches may be aggregated at an intermediate baseband switch then switched to their respective destinations. Comparatively then, a network made up of carrier switches would appear to be not as efficient as a network made up of baseband switches.
Fine granularity in a carrier switch may be realized using a large number of carrier signals, each carrier signal modulated by a baseband signal of a narrow bandwidth. For example, instead of using a multiplex of 16 carriers, each modulated at 10 Gigabits per second, a multiplex of 16,000 carriers, each modulated at 10 Megabits per second, may be used. The modulated 16,000 carrier signals may then be spatially switched to separate the individual carrier signals which would be demodulated only at the receiving end switch. Exchanging carrier signals among 64 such carrier multiplexes would be a challenging task. Another way of realizing fine granularity is to use time-division multiplexing (TDM), where a carrier signal can be time sliced into time-limited signals organized in a TDM frame of 1,024 time slots for example. Thus, a carrier modulated at 10 Gb/s, can be divided into 1,024 units of about 10 Mb/s each. Exchanging carrier signals among 64 multiplexes of 16 carriers each requires a fast space switch having 1,024 input ports and 1,024 output ports. A fast modular space switch of this capacity is realizable. However, aligning the TDM frames at the 64 input ports is not assured without the use of input buffers. With the current state of the art, a carrier switch can not include input buffers. There is no practical facility to store carrier signals and retrieve them in any desired order. This precludes time switching without some means for coordinating the switching functions at each carrier switch with the switching functions at the preceding and succeeding switches.
In applicant's U.S. patent application Ser. No. 09/286,431, filed on Apr. 6, 1999, and titled “Self-Configuring Distributed Switch”, a method of aligning signals at the input ports of a bufferless switch receiving carrier signals from a plurality of edge nodes is disclosed. The method requires that each edge node be provided with random-access buffers. Time switching requires TDM-frame alignment at the input ports. Hence, time switching can not be used for fine-granularity carrier switching if a connection requires traversing two or more carrier switches. The time-alignment process, also called “time locking” is further extended in applicant's U.S. patent application Ser. No. 10/054,509, filed on Nov. 13, 2001 and titled “Time-Coordination in a Burst-Switching Network”.
Clearly, an ideal compact and efficient network would be based on maximizing the use of fine-granularity carrier switching in order to minimize the number of traversed switch nodes per connection. As discussed above, fine-granularity carrier switching is only feasible in a first carrier switch in a path from a source of a carrier modulated with a signal to a destination, or sink, for the signal. Consequently, fine-granularity carrier switching is used where only one switch node is to be traversed and coarse-granularity carrier switching is used where more than one switch node is be traversed. There is a need for a flexible network that combines both fine and coarse carrier-switching granularity.